In many spinal fixation systems, a spinal rod is used to correct and stabilize the spine, so that fusion can be achieved at selected levels in the spine. The spinal rod is attached to the spine by a series of screw or hook implants, with each implant attached to a vertebral body. Typically, each implant includes a head portion, sometimes called a “rod receiver” or “saddle”, that forms a channel. The rod rests inside the channels and interconnects the implants to one another. A locking element, such as a set screw, is inserted into each channel and over the rod to lock the rod in place in the channel. The rod is thereby supported by multiple implants that are secured to the rod at multiple locations.
In many instances, the rod does not align precisely with all of the saddles during insertion. One common reason for this is that the implants and their channels are seldom arranged relative to one another in a perfectly straight line. More often, the implants are positioned in a non-linear arrangement due to the curvature of the spine and the relative positions of the vertebrae. As a result, when one section of a rod is locked down in one implant, another section may extend above the next implant, outside the channel. For example, after a first section of a rod is locked in place in one saddle, a second section of the rod may extend partially outside or entirely outside the channel in the next adjacent saddle. This is particularly common when the receiver bodies are small, with relatively short channels. When this happens, the second section of rod must be pushed down into the saddle until it is reaches a sufficient depth within the channel to provide room for a set screw above the rod. A rod persuader instrument may be used in such cases to advance the second section of the rod down into the channel, so that a set screw or other fastener can be inserted into the saddle over the rod.
Rod persuader instruments that are used to perform rod reduction must apply sufficient downward force on the rod to position the rod in the saddle portion of the implant. To provide such force, many rod persuader instruments are designed to attach directly over the opening in the saddle. This attachment can be difficult, especially when the rod receiver body has a relatively short profile length. When the rod receiver body has a relatively short length, there is little or no surface area for gripping by a rod persuader.
Another drawback with instruments that attach over the top of a rod receiver body is limited visibility. Rod persuader instruments that attach over top of a receiver body can visually obstruct the opening in the saddle, and the surgeon can not see down into the channel. This can make it difficult to introduce a set screw into the saddle.
In view of the foregoing, many known rod persuader instruments leave much to be desired in terms of function and operation.